I've had the girls for about three months now, with little to no problems, but recently I've notice the fur near Natsuki's eyes has been thinning, mostly on the right side. It is a bit purple, which is concerning. The only other difference I've noticed is she doesn't come out from hiding as much, but apart from that she seems fine, no scratching so I don't think it's mites. Am I worrying too much or is something wrong? I really hope she is okay. Here is a picture, on the side it is worse:

Usually mice overgroom when they are bored. If your cage isn't very packed full of toys, you can try to pack it with more fun things to do and explore. I had a trio of girls in a 10 gallon, and one girl started to overgroom her sisters, but when I moved them to a bigger tank with more things to do, all of their fur grew back and the overgrooming totally stopped, so I know a cage size upgrade can help a lot too, if your cage isn't very big already Just try to keep her entertained!

Edit to add: It could also be that Luana is trying to show dominance over the others, which overtime might stop as she gets older and isn't the dominant mouse, or alpha, anymore, but if your girls seem to get along well then it might not be likely that it's a dominance thing.

Thank you so much for your help, I was planning on buying/making some more toys already, but now I'll make sure that the tank is full of fun things! Size isn't really a problem, they are already in a tank a little bigger than a 20 gallon. Also, dominance could be apart of it. Luana does mount the others and start squabbles occasionally, but most of the time they get along just fine.